Energy balance - Heat recovery at Finnish pulp mill

Energy self-sufficiency

UPM-Kymmene has been working toward energy self-sufficiency for the past 10 years. The Kaukas pulp mill underwent a large extension and modernization program at the end of the 90s and is since then considered energy self-sufficient – a goal achieved by recovering secondary heat from cooling processes for reuse elsewhere in the plant. Asko Paakki, operations manager of Kaukas Pulp Mill, estimates that between 10 and 20 percent of energy recovered at the mill is used elsewhere on the site. “The energy surplus is sold, usually in the form of process steam to the paper and especially the saw mills,” he says.

“Energy recycling requires intricate planning and a holistic approach”

Such recycling requires intricate planning and a holistic approach to cooling needs and recovery possibilities throughout the entire plant.

It is a balancing act. Secondary heat – by definition heat transferred from primary heat sources to process flows – is recovered in cooling and condensation waters, using a system of heat exchangers, and recycled through the plant. More than 100 heat exchangers transfer secondary heat from where it is not required to where it is required, creating an energy balance in the mill that saves Kaukas millions of euros in energy costs each year.

Industry trend

The trend in the pulp and paper industry is to develop processes toward effluent-free operation to reduce wastewater flows and to improve process efficiency. This means higher wastewater temperatures which require additional cooling, explains Pekka Svinhufvud, manager of energy economics at UPM-Kymmene’s Kaukas Pulp Mill.

Svinhufvud says the energy-saving possibilities of process integration are very plant-specific. “They depend on the process concept, the mill layout and the energy and water balance of the plant. Paakki explains that process effluent must be cooled from 70 to 80 degrees Celsius (158 to 178 degrees Fahrenheit) to below 40° C (104° F) before it goes to the biological effluent treatment plant, because the micro-organisms will not survive higher temperatures. At Kaukas, this heat is recovered using natural water in a heat exchanger – an alternative to the cooling towers used in many pulp and paper mills, where heat is simply released into the atmosphere.

In the pulp mill, cooling water from the evaporation plant, filtrates from the bleaching plant, hot water from cooling and warm water from the gas flue scrubber have the most potential for heat recovery.

“When we commenced our cooperation, testing and trials were conducted with a couple of Alfa Laval heat exchangers. After two years of satisfactory results, we bought more and more. We are continuously looking for opportunities to recover heat. We are still changing the manufacturing process and are now looking at an area between inter-filtrate exchange.” - Asko Paakki, operations manager of Kaukas Pulp Mill

Heat exchangers at core of heat recovery

A completely new heat recovery system with WideGap plate heat exchangers redesigned to gain higher temperatures has been installed at the UPM-Kymmene Kaukas Mill in Finland to allow recovery of secondary heat with Alfa Laval as the main supplier. Alfa Laval also redesigned 13 spiral heat exchangers to increase efficiency.